346 



ANIMAL METABOLISM 



Higher animals also are able to convert glutamic acid into proline, 

 hydroxyproline, and ornithine, as follows: 



COOH 



I 

 H,NCH 



H2NCH 



I 



CH2 



I 



CH2 



I 



CH2 



I 



NH2 



L-Ornithinc 



The ornithine produced is readily converted into two additional amino 

 acids, citrulline and arginine (see urea formation, p. 352). Although 

 arginine can thus be synthesized in the animal, the rate of production 

 is often too slow to meet bodily needs. It has been shown, for example, 

 that the growth of rats fed a ration lacking arginine is greatly stimu- 

 lated by adding this amino acid. 



Tyrosine is another amino acid which is synthesized in the animal 

 body, the precursor being phenylalanine: 



COOH 



COOH 



H2NCH 

 CH, 



H2NCH 



(50) 



L-Phenylalanine 



OH 

 L-Tyrosine 



Accordingly, it has been found that tyrosine has a sparing action fur 

 phenylalanine, just as cystine has for methionine. 



